Cooling Towers: How do they Work? Function & Working Principle

1. Evaporative cooling towers: definition and working principles

The section below will attempt to answer the following most frequently asked questions.

• “What are evaporative cooling towers (or simply cooling Towers), what do they do and how do they work?”
• “Where are they used, and why? Why are they necessary?”
• “What is the definition of ‘cooling tower’ or ‘evaporative cooling tower’?”
• “What does ‘evaporative’ mean?”

1.1. What are Cooling Towers?

Evaporative cooling towers, or cooling towers are devices which make use of a natural principle which is as simple as it is effective: the forced evaporation of a minimum quantity of water, compared to the main mass, takes place by the dissipation of heat from the mass itself; the mass, therefore, cools down (latent heat of evaporation).

The process by which evaporative cooling occurs is as simple as it is ancient: the archaic amphorae made of terracotta, a porous material, allowed the water to leach outwards in minimal quantities. In this way, an evaorative process took place: the water inside was kept cool even in high ambient temperatures.

1.2. How do Cooling Towers work? Exploiting the latent heat of evaporation

Evaporative cooling towers are able to best realise the water/air heat exchange process: evaporation is forced through the use of simple, effective components which normally require minimum maintenance.

To better understand how heat dissipation occurs, two concepts need to be introduced.

• Sensible heat. This is the amount of heat energy that is added to, or subtracted from, a physical element (such as louvers) to change its temperature.
• Latent heat. This is basically founded on the change of state that a substance can undergo as a result of heat addition or loss. In the case of water, it can change from a liquid phase to a solid phase (ice) if heat is removed when it reaches freezing point. It can also change from a liquid phase to a gaseous phase (vapour) if heat is added when it reaches boiling point. Latent heat is therefore defined as heat that is introduced or removed to change the state of the water. Specifically, in evaporative cooling systems, it is defined as latent heat of evaporation.

A well-designed evaporative tower is able to provide the water with as much air contact surface area as possible, in order to optimise the latent heat exchange.

To make this heat exchange possible, the evaporative tower must therefore be able to offer a very high air/water contact surface. This is achieved by means of a heat exchange surface, specifically designed for the purpose, and a fan able to move a defined volume of air according to precisely defined parameters. We will now look at the importance of these internal components in detail.

1.3. The wet bulb temperature

An important physical concept makes it possible to have a better understanding of how cooling towers work: the wet bulb temperature is fundamental to the theory of how all evaporative systems and, more specifically, cooling towers work.

Practically speaking, this parameter precisely defines the “worst” conditions of temperature and relative humidity in the area of installation. It provides a precise reference for theoretically achievable output temperature of the evaporative tower.

1.4. Efficiency of cooling towers

Given their simple construction, combined with the high levels of efficiency in terms of cost/dissipated kW ratio, evaporative cooling towers are still the most commonly used cooling device in both HVAC and, above all, industrial environments: there are no particular moving parts, except a fan (which can be positioned at both the suction and delivery points). On the other hand, electricity use is truly low when compared to other systems used for the same purposes.

This is especially true where large heat amounts need to be dissipated (e.g. steelworks, chemical plants, power plants) as cooling towers are unrivalled in terms of the electrical power used and the minimum space required for their installation.

Not to mention that the achievable temperatures, with regard to cooled water, are well below the ambient temperature: unlike louver systems, for example, which are bound by this limit. This is due to the fact that the evaporative systems work by exploiting the latent heat of evaporation (the minimum achievable limit of the water is the wet bulb temperature).

1.5. A comparison of cooling technologies: evaporative, dry, adiabatic and mechanical coolers

When a cooling system for industrial processes or HVAC is to be built, several fundamental points must be considered: as a result, we ca select the most suitable system for our plant. Specifically, the decision must take into account both the required operating temperatures and the environmental conditions at the site of installation.

For example, if a temperature of the cooled fluid below ambient temperature is required, an evaporative system would be preferable: in this case, the minimum theoretical limit of the cooled fluid is, as we have seen, the wet bulb temperature of the air.

On the other hand, dry systems are based on the sensible exchange which is much less efficient than the exchange of latent heat of evaporisation. The limit in this case is imposed by the temperature of the cooling fluid, namely the ambient air. Where it is sufficient to cool the fluid to a temperature above ambient temperature, an air cooler should be used.

A third option is to design an adiabatic system where a fluid temperature equal to or slightly lower than the ambient temperature is desired.

All this helps to show that there is no such thing as a “good for every season” cooling system: making the right choice, based on the design requirements and environmental conditions, means optimising energy use, reducing the required space and ensuring the systems are in conditions where they can operate at their best.

Chiller units are a separate subject: in this case, however, we are talking about devices that use specific mechanical components to achieve cooling (compressors, evaporators), instead of “natural” elements such as air or water.